Quantifying the sources of dissolved inorganic carbon within the sulfate-methane transition zone in nearshore sediments of Qi'ao Island, Pearl River Estuary, Southern China  

作　　者：WU ZiJun ZHOU HuaiYang REN DeZhang GAO Hang LI JiangTao 

机构地区：[1]State Key Laboratory of Marine Geology, Tongji University

出　　处：《Science China Earth Sciences》2016年第10期1959-1970,共12页中国科学（地球科学英文版）

基　　金：supported by the State Key R&D Project (Grant No. 2016YFA0601100);the National Natural Science Foundation of China (Grant Nos. 91428207 & 41176065)

摘　　要：The significance of the various biogeochemical pathways that drive carbon cycling and the relative fractions of dissolved inorganic carbon(DIC) produced by these reactions within the sulfate-methane transition zone(SMTZ) are still being debated. Unraveling these processes is important to our understanding of the benthic DIC sources and their contributions to the global carbon cycle. Here, we measure pore water geochemistry(chlorine, sulfate, methane, Ca^(2+), Mg^(2+), DIC and δ^(13)C-DIC) as well as solid geochemistry(sedimentary organic carbon(SOC) and δ^(13)C of SOC) in nearshore sediments from Qi'ao Island in the Pearl River Estuary of the Southern China Sea. Our analysis indicates that SOC originates from the mixing of carbon from terrestrial and marine sources, and that terrestrial materials dominate the net loss of SOC during the degradation of organic matter, especially at sites located near the river outlets. Sulfate reduction via SOC degradation is not appreciable in the upper sediment layer due to conservative mixing-dilution by freshwater. However, below this layer, the anaerobic oxidation of methane(AOM) and methanogenesis occur. Within the SMTZ, the δ^(13)C mass balance shows that the proportions of DIC derived from organoclastic SO_4^(2-) reduction(OSR) and AOM are 50.3% to 66.7% and 0.1% to 17.9%, respectively, whereas methanogenesis contributes 17.0% to 43.9%. This study reveals that the upward diffusion of DIC from ongoing methanogenesis significantly influences carbon cycling within the SMTZ in these estuarine sediments. As a result, we suggest that the plots of the ratio of change in sulfate to change in DIC in pore water should be used with caution when discriminating between sulfate reduction pathways in methane-rich sediments.The significance of the various biogeochemical pathways that drive carbon cycling and the relative fractions of dissolved inorganic carbon （DIC） produced by these reactions within the sulfate-methane transition zone （SMTZ） are still be- ing debated. Unraveling these processes is important to our understanding of the benthic DIC sources and their contributions to the global carbon cycle. Here, we measure pore water geochemistry （chlorine, sulfate, methane, Ca^2＋, Mg^2＋, DIC and δ^13C-DIC） as well as solid geochemistry （sedimentary organic carbon （SOC） and δ^13C of SOC） in nearshore sediments from Qi＇ao Island in the Pearl River Estuary of the Southern China Sea. Our analysis indicates that SOC originates from the mixing of carbon from terrestrial and marine sources, and that terrestrial materials dominate the net loss of SOC during the degradation of or- ganic matter, especially at sites located near the river outlets. Sulfate reduction via SOC degradation is not appreciable in the upper sediment layer due to conservative mixing-dilution by freshwater. However, below this layer, the anaerobic oxidation of methane （AOM） and methanogenesis occur. Within the SMTZ, the δ^13C mass balance shows that the proportions of DIC de- rived from organoclastic SO42- reduction （OSR）and AOM are 50.3% to 66.7% and 0.1% to 17.9%, respectively, whereas methanogenesis contributes 17.0% to 43.9%. This study reveals that the upward diffusion of DIC from ongoing methanogene- sis significantly influences carbon cycling within the SMTZ in these estuarine sediments. As a result, we suggest that the plots of the ratio of change in sulfate to change in DIC in pore water should be used with caution when discriminating between sul- fate reduction pathways in methane-rich sediments.

关 键 词：Dissolved inorganic carbon Sulfate reduction METHANE Carbon isotopes Estuaries 

分 类 号：P736.21[天文地球—海洋地质]